Annular light beams are used, for example, for illuminating an object with impinging light in a microscope. In such an arrangement, light of a cold light source is guided through a light guide, generally formed by a multitude of individual fibers, to the annular illumination system and its annular light emanating surface which is arranged around the lens of the microscope. The light guide or the fibers of it extend through a concentric ring-shaped gap, and are distributed uniformly over the area of this gap. When producing and machining the emanating surface of light guiding fibers, the fibers of the light guide pass the ring-shaped gap inclined to the rotational axis of the annular illumination arrangement and of the microscope lens, and are polished parallel to the plane of the gap.
By this geometrical configuration the cone of the emanating light is asymmetrical to the symmetry or rotational axis of the annular illumination system due to the manufacturing conditions, and the object field is illuminated homogeneously from a minimum working distance of 20 mm on.
With a working distance longer than about 300 mm, however, more and more beams of the emanating light cone leave the range near the axis and, therefore, do not contribute to object illumination. This is a great disadvantage, because the illumination intensity, thus, decreases with increasing working distance.
It has been suggested already to avoid this disadvantage in that some fiber groups of the annular illumination system are pivotable or displaceable with respect to the axis of symmetry in order to be able, in the case of longer object distances, to focus onto the object. This known approach, however, involves an expensive mechanical construction and, moreover, does not result in a homogeneous distribution of light intensity, as required, because the number of fiber groups arranged at the periphery of the annular illumination system.